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The Young Brain

Submitted by Rowden on Thu, 10/06/2011 - 22:22

David Dobbs 2011

Parents often find themselves unable to understand why youngsters, not only teenagers but also young adults into their early and mid 20’s, act the way they do. They often seem quite arbitrarily ready to take life-threatening or life-changing risks, without seeming to evaluate logically what can happen. In the late 20th century brain-imaging technology was developed and researchers were able to track both the physical development and the patterns of activity. The results were surprising. The brain takes much longer to develop than most scientists had thought.

The first full series of scans of the developing adolescent brain in the 1990’s showed that the young brain undergoes a massive reorganisation between the age of 12 and around 25. The brain doesn’t actually grow much, it has already reached 90% of its full size by the age of 6 years, but it does undergo an extensive remodelling, resembling a network and wiring upgrade.

The axons (nerve fibres used in inter-neuron signalling) become more insulated with myelin, boosting the transmission speed by up to a hundred times. The dendrites (branchlike extensions the neurons use to receive signals from nearby axons) grow twiggier and branch out more and the most heavily used synapses (the chemical junctions across which axons and dendrites pass messages) grow richer and stronger. Synapses which see little use begin to wither. This synaptic ‘pruning’ causes the brain’s cortex (the outer layer of grey matter where we do much of our conscious and complicated thinking) to become thinner and more efficient.

This process of ‘maturation’, once thought to be completed by mid-teens, continues throughout adolescence. Imaging work done since the 1990’s shows that these physical changes move in a slow wave from the rear of the brain to the front, from older and basic areas of the brain (which control base functions such as vision, movement and fundamental processing) to the evolutionary newer and more complicated thinking areas in the front. The corpus callosum, which connects the left and right side of the brain and carries traffic essential to many advanced brain functions, steadily thickens. Stronger links also develop between the hippocampus (like a memory directory) and the frontal areas which set goals and weigh different agendas. As a result we get better at integrating memory and experience into our decisions. At the same time the frontal areas develop greater speed and richer connections allowing us to weigh far more variables and agendas than before.

If this development proceeds normally we get better at balancing impulse, desire, goals, self-interest, rules, ethics, even altruism, generating behaviour that is more complex and often more sensible and logical. At times however and at first, the brain does this work clumsily and all the new cogs don’t always mesh together.

Tests at the University of Pittsburg during brain scans of children, adolescents and students in their early twenties have shown that where the instruction in the test is to ignore certain distractions the performance depends very much on age. Ten to 12 year olds fail about 45% of the time, teens do better and even at as young as 15 youngsters can perform almost as well as adults if they are motivated. However what was more interesting was not the scores but the scan results during testing. Compared with adults, teens tended to make less use of brain regions that monitor performance, spot errors, plan and stay focused – areas the adults bring online quite automatically. This let the adults use a variety of brain resources and better resist temptation, while the teens used these areas less often and more readily gave in to impulses. By early twenties the brain responds to tasks much as the adult brain does, indicating that the improvement is as a result of richer networks and faster connections coming online and rendering the executive region more effective.

Such studies help explain why adolescents and those in their early 20’s often behave with such vexing inconsistency. There has been an explosion of scientific papers and articles about the ‘teen brain’ which presents this as ‘a work in progress’ or ‘a less than mature state of development’. However this does not necessarily tell the full story.

Over the past 5 years or so a number of researchers have begun to view brain and genetic findings in a more flattering light, one coloured by evolutionary theory. That the teen to early 20 year old is not a ‘rough draft’ but just another biological animal being programmed and adapted for the purpose of leaving the safety of home and moving into the complicated world outside. This sits well with biology’s most fundamental principle, that of natural selection. This is a period in the ‘near adult’ life when the human organism is entering a highly functional and adaptive period.

Start with the teen’s love of the thrill. New and exciting things are never valued more highly than in adolescence. Sensation seeking can lead to dangerous behaviours but can also generate positive ones, like creating a wider circle of friends, which generally makes us healthier, happier, safer and more successful. This upside probably explains why openness to the new with all its dangers remains a highlight of adolescent development. The love of novelty leads directly to useful experience.

Also peaking during adolescence is ‘risk-taking’. Risk is courted more avidly in the teens than at any other time. The period from 15 to 25 years of age brings highs and lows in all sorts of risky ventures with in many cases ugly outcomes. This age group dies of accidents of almost every sort and at a high rate. So are these kids just being stupid? Actually not. Adolescents usually reason their way through situations just as well as adults and understand too that they are mortal. Teens take more chances not because they don’t understand the dangers but because they weigh risk versus reward differently. In situations where risk can get them something they really want, they value the reward much more heavily than the adults would. Often too this aspect is accentuated when youngsters are together in a group, instead of alone or with adults.

Researchers such as Steinberg and Casey believe this risk-friendly weighing of cost versus reward is selected because, over the course of human evolution, the willingness to take risks has given an adaptive edge.

Another trait that marks adolescence is the preference for the company of those of their own age more than ever before or after. Teens offer far more novelty than the familiar old family does. But teens understand that we enter a world made by our parents but they will live most of their lives in one run and remade by people of their own age. Knowing, understanding and building relationships with them bears critically on later success. This supremely human characteristic makes peer relations not merely a sideshow but the main show in town. At a neural level social rejection is a threat to existence itself!

Excitement, novelty, risk, the company of peers, may all seem to just add up to nothing more than doing new stuff with friends. Look deeper and you see that these traits that define adolescence make us more adaptive both as a species and as individuals. Broadly defined these traits show themselves in virtually all human cultures, modern or tribal. This period’s uniqueness rises from genes and developmental processes that have been selected for over thousands of generations because they play an amplified role during this key transitional period: producing a creature optimally primed to leave a safe environment and move into unfamiliar territory. This move outward is not only the most difficult thing that humans do as well as the most critical – not just for individuals but for a species that has shown an unmatched ability to master challenging new environments.

One final key to both the clumsiness and yet remarkable adaptability of the teen brain is the prolonged plasticity of those late-developing frontal areas as they slowly mature. These areas are the last to lay down the fatty myelin insulation which speeds transmission. If we need these areas for the complex task of entering the big wide world why aren’t they up to speed when the challenges are most daunting? The answer is that speed comes at the price of flexibility. When the myelin insulation is laid down it’s as if the wiring is getting upgraded, but once it’s done it’s harder to change.

This delayed completion (the forebrain’s myelination during the late teens and early 20’s) – a withholding of readiness – heightens flexibility just as we confront and enter the world that we will face as adults. This long, slow, back-to-front developmental wave, completed only in the mid-20’s, appears to be a uniquely human adaptation and may be one of our most consequential.